Tropical fires represent a highly uncertain
source of atmospheric methane (CH<sub>4</sub>) because of the variability of fire
emissions and the dependency of the fire CH<sub>4</sub> emission factors (g kg<sup>−1</sup> dry
matter burned) on fuel type and combustion phase. In this paper we use new
observations of CH<sub>4</sub> and CO in the free troposphere from the Aura
Tropospheric Emission Sounder (TES) satellite instrument to place
constraints on the role of tropical fire emissions versus microbial
production (e.g. in wetlands and livestock) during the (October) 2006 El
Niño, a time of significant fire emissions from Indonesia. We first compare
the global CH<sub>4</sub> distributions from TES using the GEOS-Chem model. We
find a mean bias between the observations and model of 26.3 ppb CH<sub>4</sub>
that is independent of latitude between 50° S and 80° N, consistent with previous validation studies of TES CH<sub>4</sub> retrievals
using aircraft measurements. The slope of the distribution of CH<sub>4</sub>
versus CO as observed by TES and modeled by GEOS-Chem is consistent (within
the TES observation error) for air parcels over the Indonesian peat fires,
South America, and Africa. The CH<sub>4</sub> and CO distributions are correlated
between <i>R</i> = 0.42 and <i>R</i> = 0.46, with these correlations primarily limited
by the TES random error. Over Indonesia, the observed slope of 0.13
(ppb ppb<sup>−1</sup>) ±0.01, as compared to a modeled slope of 0.153 (ppb ppb<sup>−1</sup>)
±0.005 and an emission ratio used within the GEOS-Chem model of
approximately 0.11 (ppb ppb<sup>−1</sup>), indicates that most of the observed methane
enhancement originated from the fire. Slopes of 0.47 (ppb ppb<sup>−1</sup>) ±0.04
and 0.44 (ppb ppb<sup>−1</sup>) ±0.03 over South America and Africa show that the
methane in the observed air parcels primarily came from microbial-generated
emissions. Sensitivity studies using GEOS-Chem show that part of the
observed correlation for the Indonesian observations and most of the
observed correlations over South America and Africa are a result of
transport and mixing of the fire and nearby microbial-generated emissions
into the observed air parcels. Differences between observed and modeled
CH<sub>4</sub> distributions over South America and southern Africa indicate that
the magnitude of the methane emissions for this time period are inconsistent
with observations even if the relative distribution of fire versus biotic
emissions are consistent. This study shows the potential for estimation of
CH<sub>4</sub> emissions over tropical regions using joint satellite observations
of CH<sub>4</sub> and CO.